You need to be a little more specific. There are three separate versions of MacOS 9: MacOS 9.0, MacOS 9.1, and MacOS 9.2. Not counting the public beta, there are five versions of MacOS X 10, MacOS X 10.0, MacOS X 10.1, MacOS X 10.2, MacOS X 10.3, and MacOS X 10.4. This is important because the system utilities changed over time. You also need to make sure that you have the shrinkwrap versions of the OS unless you have the specific version that shipped with a particular computer.
I recommend that you format your hard drives using the most recent version of MacOS 9 and that you choose HFS+. Use the MacOS 9 utility, Drive Setup. Install your most recent version of MacOS 9. Use Software Update to install all available updates. Then, install the most recent version of MacOS X over MacOS 9. This will allow you to boot either OS. Use the MacOS X version of Software Update to install all available updates.
You should have at least 256 MB RAM. You should also have, as a bare bones minimum, 6 MB of hard drive space. For good performance, you should have much more than 6 MB.